This is based on his behavior after he is made king unhardened, always running away and leaving the ruling to Anora/Eamon or the warden. That's not idle speculation, that's in the game--at least the end slides.
The end slides also say that Cullen becomes a deranged serial killer if you don't Annul the Circle. 
I think the devs have also said that the slides are just in-universe rumors from characters who don't have all the facts, and that they shouldn't be taken as "canon" so much as a possible "what could have happened" after the events of the game until proven otherwise, but should be taken with a grain of salt. So, I take them with a grain of salt.
As for it not having an impact on future games, well constraints of storytelling had to be applied to his cameo. They weren't going to create 2-3 different scenes based on whether hardened or not just to satisfy every single contingency from Origins with multiple lines they have to pay Steve Valentine to deliver. Plus neither Hawke nor the Inquisitor is someone Alistair feels comfortable being himself with as they have only just met.
Is that what the devs said? If not, I think it's speculation. ![=]](https://lvlt.forum.bioware.com/public/style_emoticons/default/sideways.png)
Personally, I wondered why they didn't have a "hardened" or "unhardened" Alistair in Inquisition too, and I speculate they don't do it partly because he gained confidence as he got older, so it doesn't really matter in the long run.
During DAO, it should be noted that Alistair is only 18-20, and very much a sheltered boy who was told what to do his whole life, and is out on his own for the first time. He was raised in a castle (always told what to do by Eamon), then sheltered in the Templars (always told what to do by his superiors), then spent 6 months with the Grey Wardens (always directed where to go by Duncan), then was thrust into a leadership role with you after the Grey Wardens died. Of course he doesn't have confidence in his leadership or decision-making abilities; he's never had to before.
But by Inquisition, he's had ten years to "grow into his armor," so to speak.
If made king, he's had ten years to get used to wearing the crown and dealing with affairs of state. (Maybe he did start off shirking his responsibilities and running away to taverns like the slides said, or those were just exaggerated rumors by subjects who loved him, but he grew up and got over it by Da2/Inquisition). If a Grey Warden, he's had ten years to get used to working with the Grey Wardens as an equal or superior (not as a Junior Warden) since you met him at Ostagar.
If you didn't harden him during Origins (when he was still an inexperienced "lad"), then he hardened into the role of his own accord and grew up by the time the Inquisitior (possibly) meets him in Inquisition.
What I don’t understand about Anora is why she allows her father to be the regent? Isn’t she the Queen? Shouldn’t she be the regent? It’s like his father wasn’t enough confident on her about ruling Ferelden, but she claims that everybody knows she’s been the one who has been taking decisions instead of Cailan. If Loghain is the general of the armies, why taking power this way? Wouldn’t have been a better move, keep Anora as the regent, and he governing from behind the throne?
Um, Anora can't be regent because (according to the Merriam-Webster Dictionary) a regent is "a person who governs a kingdom in the minority, absence, or disability of the sovereign." The only way Anora would be regent is if someone else was the sovereign king or queen for her to be the regent of, and we all know Anora (and Loghain, for that matter) will not give up her throne.
Otherwise, I completely agree.
Loghain goes on and on about how Ferelden has a capable queen and he "just commands her armies," yet he crowned himself king in all but name, took over the politics (and did a terrible job!), then brushed her aside when she disagreed with him. (It's not like she'd know more about politics than he does; why listen to her advice?)
The only reason I can think that she let him declare himself regent is that she thinks her daddy walks on water. Like Ser Cauthrien, she's blindly devoted to Loghain and thinks he can do no wrong. It takes him nearly driving the country into the ground for her to realize he's not the perfectly flawless "never-had-a-bad-idea-in-his-head" superhero that she thought he was.
I totally agree with it. I see Alistair like the kind of person who has potential but he doesn’t believe enough on himself and only needs a little push to fulfill it, I see that if we harden him we do that little push, like if it were something that already is inside him and you help it to flourish, and that’s why I harden him, and however he marries my warden, I don’t want him to be only a puppet of her or Eamon about how he should rule. I see as he can have them as advisors and can consider their opinions but he shouldn’t allow them to take all the decisions.
I personally don't like hardening Alistair.
I don't see it as "raising his self-esteem" or "giving him that little push to believe in himself," I see it as crushing his spirit. You're kicking him while he's at his lowest point from being rejected by the last family he thinks he has. Rather than reassuring him that you're his friend or that people care about him, you're basically confirming his fears that no one will ever love him or care about him because they're all out for themselves (including you), so he should just be out for himself too. I hated that!
I hardened Alistair once and I hated it! He was always so angry and grumpy, and always convinced that everyone had some secret ulterior motive and were trying to use you and him, even if it was clearly not the case. For all his faults, unhardened Alistair is cheerful and sees the best in everyone. Hardened Alistair is always angry or grumpy and just suspects the worst in everyone.
I personally don't think he's happier or more confident if hardened, he's just convinced (by you) that everyone's a self-serving jerk so he should be a self-serving jerk too (even though it goes against his own morals), which he sometimes has to visibly rationalize to himself in front of the Warden.
On the other hand...
However I always forget that Alistair in that moment is very young, and hardened or not, when we see and older version of him in DA:I, being king or mainly been a warden, he seems more confident an able to act on his own without waiting for orders, so it also comes with age and experience
I agree. At the end of the game, Alistair is just young and inexperienced and not very confident in himself. I think by Inquisition he's had time to grow some confidence (if unhardened) or mellow out (if hardened), and basically be a better-rounded adult. (Not too much of a push-over like unhardened Alistair, but also not a suspicious grump like "hardened" Alistair.)
I may not agree with you guys about hardening Alistair, but in the long run I don't think it really matters because he grows up and becomes a more confident and well-rounded adult whether you harden him or not. (Unless he's a wandering drunk, but then Teagan came and got him in DA2, so he's probably on the recovery train.)